The Earth Story

The Great Unconformity in the Grand Canyon

We’re finally here. I’ve literally spent a couple weeks in a small slice of the Grand Canyon. The basement rocks and the Grand Canyon Supergroup rocks make up a small piece of the Western U.S. They tell hugely important stories of how the continent was assembled, but compared to the units we’re about to see they appear as but a sliver.

Everything we’ve seen so far shows up in small areas in the inner gorge. Suddenly in literally 1 plane, everything is going to change and we’re going to look at rocks that outcrop across the entire Western United States.

Each of these photos shows the same boundary; generally it is called the Great Unconformity (although that term is also used for Hutton’s unconformity in Scotland). It is so obvious that it was recognized as a major boundary during John Wesley Powell’s first trip through the Canyon in the late 1800’s.

The Great Unconformity is an erosional boundary key to the history of the Western U.S. When we last left the rocks of the Grand Canyon Supergroup, they were being folded and pushed upwards. Sediments deposited below the waters were giving way to conglomerates deposited by rivers.

Most of the sedimentary rocks in the world are deposited when the rocks move below the water line. In the ocean it’s easy for sediments to accumulate and stay there. Above the water line, sediment tends to be eroded and washed out to the ocean, where it finally comes to rest. Once the rocks of the Grand Canyon Supergroup were exposed at the surface, they started to erode.

This process was repeated throughout the Western U.S. After Rodinia broke apart, there were large sedimentary units deposited in basins, but deposition in those basins stopped once sea levels dropped and this area was exposed at the surface. This created an erosional boundary that cuts across much of the continent - the Great Unconformity.

In the Grand Canyon, only a small slice of those sediments are preserved. Along one normal fault there is a single block of the Grand Canyon Supergroup rocks and that’s it. In most areas of the Canyon, whatever sediments were deposited have washed away and the rocks above sit directly on the Vishnu Schist basement.

The overlying unit is a sandstone we’ll talk about next. It even contains clasts of the Vishnu Schist and the Grand Canyon Supergroup at its base, showing how the Great Unconformity is an erosional boundary.

Western North America was pushed up above sea level for several hundred million years. It eroded and then finally found the ocean again as sea levels rose and the rocks dropped down on these normal faults.

This boundary and this story repeat throughout the Western U.S. Personally I’ve put my finger on this layer in Wyoming, Montana, and South Dakota (no that's not me in this photo, photo taken from Flickr account). This boundary occurs over thousands of kilometers. See the person putting their finger on the unconformity? At that point, her finger is on about 1.2 billion years of time.

A recent paper proposed that the Great Unconformity is the end result of a major, global process - the "Snowball Earth" events where glaciers advanced to the equator and the planet froze over for tens of millions of years. During this time, many areas of the planet were eroded, and just after that event there was a change in the isotopic composition of rocks being erupted by volcanoes. To account for the amount of mass needed, the scientist estimated that at least 3 kilometers of sediment were scraped off by the glaciers - around the entire world. This single line, running around the world, at this spot might be the entire record of one of the largest climate shifts in Earth's history.

-JBB

Image credits: https://www.flickr.com/photos/brewbooks/5216562085 https://www.flickr.com/photos/ceedave/4647303077 https://www.flickr.com/photos/ceedave/4647304033/

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Sources: http://www.lpi.usra.edu/science/treiman/greatdesert/workshop/greatunconf/ http://geoscience.unlv.edu/pub/rowland/Virtual/geology.html http://www2.nature.nps.gov/geology/education/foos/grand.pdf https://www.pnas.org/content/early/2019/01/07/1804350116

The Cardenas Basalt

The last several units in the Grand Canyon have been sedimentary rocks. These sandstones and shales were deposited in basins created as normal faults grew in the area that would one day be the Grand Canyon. On top of the Dox formation, and occasionally even penetrating some of the lower rocks we suddenly find a different rock type: the Cardenas basalt.

The Cardenas is the black unit seen in this image from Cardenas creek. This is an igneous rock, produced by a series of volcanic eruptions around 1.1 billion years ago. Because it is an igneous rock it can be well dated and modern techniques uniformly give that age. The stratigraphy suggests a rapid outpouring of lava started in this area at the end of the Dox formation. There are locations in the upper portion of the Dox suggestive of interactions between basalt and the sediments; sediments that were pushed around or altered by the heat of the lava on top, so the lava must have come in at the end of deposition of the Dox formation.

The Cardenas basalt is thick when it is found in the canyon: up to 300 meters thick. This thickness implies that the location where it erupted is somewhere very near the canyon as deposits of lava usually become thinner the farther from their source they are.

The Cardenas formation has 2 members; an upper and lower portion. The lower portion is represented by olivine rich basalts that have textures like Pahoehoe found within. The upper unit is slightly higher in silica, reaching andesite compositions. The dikes, as seen here, of magma that intrude the lower sediments match the Cardenas lavas in composition, indicating that they have a similar source.

At about this time, 1.1 billion years ago, vast outpourings of lava are found in rocks of the Grand Canyon, Death Valley, and far to the north in Montana’s Belt basin. This continent-wide outpouring of lava probably helps indicate its cause. Some scientists have suggested that the lavas could have been produced by a large mantle plume rising beneath western Laurentia, with its eruption made easier by the rifting processes.

The Grand Canyon Supergroup sediments were deposited in rift basins associated with normal faults and crustal extension. The same processes were occurring far to the north in the Belt Basin and in Death Valley at the same time. Crustal extension can allow the Earth’s mantle to rise towards the surface and begin melting. The presence of basalts in these basins associated with rifting and normal faulting therefore suggests that the lavas erupted as part of the continent was pulled apart or even rifted away, and the same forces which created the basins that the sediments collected in also gave rise to the Cardenas basalt.

-JBB

Image credits: https://www.flickr.com/photos/grand_canyon_nps/5477460388 http://upload.wikimedia.org/wikipedia/commons/f/f9/Grand_Canyon_Supergroup_with_basalt_dike_in_Hakatai_Shale.JPG (Creative commons licensed)

Read more: http://www.sciencedirect.com/science/article/pii/0301926894901082#

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The deep red colors of the Hakatai Shale

Moving up the Grand Canyon, on top of the Bass Formation limestone we find this unit, the Hakatai Shale, named for a type location in Hakatai Canyon.

The Hakatai shale is up to about 200 meters thick at its thickest location and consists of alternating layers of sandy mudstone, sandstones, and true mudstones. The contact between the Hakatai shale and the Bass Limestone is the first conformable contact we’ve found while moving up the Grand Canyon; the units grade into each other over a period of several meters in some locations.

The bright red colors of this unit are a feature we will find in several Grand Canyon units, but nowhere are red colors are more well expressed than here. This unit is a shale, consisting of fine-grained sediments collected in quiet waters protected from the actions of waves. Those sediments contained enough iron that when this rock is exposed to water and air it turns a deep red color and can even paint the limestone below it a reddish color when there is runoff.

Shale layers tend to not be very strong, so this unit doesn’t outcrop very well. Some of the more sand-rich layers do stand out but generally this unit appears as eroded slopes in-between the more resistant layers above and below it.

The Hakatai is divided into 3 sub-members; the lowest member consists of fine-grained, intermixed sandstones and mudstones, the middle member is the most pure shale of the sequence, and the upper layer grades into more quartz and feldspar rich sandstones.

Features like the mudcracks shown in the 2nd image suggest that although there was little wave action, these rocks were deposited in fairly shallow water, as found in tidal flats or estuaries today.

Ages have been measured for layers of ash in the Bass formation below this unit and in the intrusive igneous rocks that occasionally cross-cut it, as seen here, bracketing the deposition of this unit between 1250 and 1140 million years ago. Keep in mind though – that’s over 100 million years of time. We know it falls somewhere in that range, but 100 million years is a very long time. Particularly with sedimentary rocks that don’t grow their own grains, sometimes that’s the best we can do.

-JBB

Image credits: http://commons.wikimedia.org/wiki/File:Grand_Canyon_Supergroup_Hakatai_Shale_with_Basalt_Dike.jpg http://commons.wikimedia.org/wiki/File:Grand_Canyon_Supergroup_Hakatai_Shale_0002_-Flickr-_Grand_Canyon_NPS.jpg

Read more: http://3dparks.wr.usgs.gov/coloradoplateau/lexicon/hakatai.htm http://www.bobspixels.com/kaibab.org/geology/gc_layer.htm http://bulletin.geoscienceworld.org/content/117/11-12/1573.full

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Bass Formation

In the Grand Canyon, the oldest rocks are highly metamorphosed schists and gneisses – considered parts of the continental basement. These rocks are only reached in the inner gorge of the Colorado River, where the river has eroded through the entire sedimentary sequence of the western United States. The top of those metamorphic units is a called a nonconformity – it is a rough, erosional surface, created when the ancient metamorphic rocks were exposed at the surface for millions of years. The metamorphic rocks of the Grand Canyon formed about 2-1.7 billion years ago, then 10% of the age of the planet passed before anything else was recorded at this site. Atop those igneous and metamorphic rocks sits a small set of tilted sedimentary rocks known as the Bass Formation or the Bass Limestone. This is the first fairly pristine sedimentary rock we find while walking up the Grand Canyon. The features in the first photo are mostly stromatolites; layers of limestone and other sediments believed to be created as mats of bacteria grew on the floors of shallow, warm oceans. The second photo was taken at the Phantom Ranch Boat Launch deep in the Canyon’s inner gorge. Look around the people – all the rocks are pretty massive, there’s no obvious layering anywhere near the people. The only place where you see layered rocks is atop the ridge in the distance – those rocks are the Bass formation. All of the lower rocks are the igneous and metamorphic rocks of the inner gorge. The Bass Formation is the lowermost member of what is known as the Grand Canyon Supergroup. The Supergroup is a series of sedimentary rocks formed in the late Precambrian, exposed at the bottom of the canyon as a sequence of rocks that have been tilted and dip off to the northeast.

The Bass is a sequence of sedimentary rocks. It contains many layers of dolomite that probably originally formed in the ocean as limestone and then were altered to dolostone after they were buried, with thin layers of sandstone, siltstone, and the occasional coarse grained conglomerate. The stromatolites are fossil bacteria colonies that formed in the ocean This sedimentary sequence indicates that water levels were changing – from rivers that deposited the conglomerates to shallow ocean waters that formed the dolomite. This unit marks the first step in a transgressive sequence – water levels were rising to cover the exposed Vishnu Schist basement rocks, and those rising waters produced this rock unit.

Geologists look for layers of volcanic ash in sedimentary rocks like this one because we are easily able to produce age dates by measuring isotope ratios in those layers. An isotope measurement on this rock gave an age of 1254 million years old. After the big tectonic events that formed the crust of the western US, that’s when the next stage – the sedimentary stage – began.

-JBB

Image credit: NPS https://www.flickr.com/photos/grand_canyon_nps/6705376193/in/photostream/ https://www.flickr.com/photos/grand_canyon_nps/8229636485/

Reference: https://bit.ly/2REaVCr By the way, we’re going to be in the Grand Canyon Supergroup for a while.

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Bass Formation

In the Grand Canyon, the oldest rocks are highly metamorphosed schists and gneisses – considered parts of the continental basement. These rocks are only reached in the inner gorge of the Colorado River, where the river has eroded through the entire sedimentary sequence of the western United States. The top of those metamorphic units is a called a nonconformity – it is a rough, erosional surface, created when the ancient metamorphic rocks were exposed at the surface for millions of years. The metamorphic rocks of the Grand Canyon formed about 2-1.7 billion years ago, then 10% of the age of the planet passed before anything else was recorded at this site. Atop those igneous and metamorphic rocks sits a small set of tilted sedimentary rocks known as the Bass Formation or the Bass Limestone.

This photo was taken at the Phantom Ranch Boat Launch deep in the Canyon’s inner gorge. Look around the people – all the rocks are pretty massive, there’s no obvious layering anywhere near the people. The only place where you see layered rocks is atop the ridge in the distance – those rocks are the Bass formation. All of the lower rocks are the igneous and metamorphic rocks of the inner gorge. The Bass Formation is the lowermost member of what is known as the Grand Canyon Supergroup. The Supergroup is a series of sedimentary rocks formed in the late Precambrian, exposed at the bottom of the canyon as a sequence of rocks that have been tilted and dip off to the northeast.

The Bass is a sequence of sedimentary rocks. It contains many layers of dolomite that probably originally formed in the ocean as limestone and then were altered to dolostone after they were buried, with thin layers of sandstone, siltstone, and the occasional coarse grained conglomerate. This sedimentary sequence indicates that water levels were changing – from rivers that deposited the conglomerates to shallow ocean waters that formed the dolomite. This unit marks the first step in a transgressive sequence – water levels were rising to cover the exposed Vishnu Schist basement rocks, and those rising waters produced this rock unit.

Geologists look for layers of volcanic ash in sedimentary rocks like this one because we are easily able to produce age dates by measuring isotope ratios in those layers. An isotope measurement on this rock gave an age of 1254 million years old. After the big tectonic events that formed the crust of the western US, that’s when the next stage – the sedimentary stage – began.

-JBB

Image credit: NPS https://www.flickr.com/photos/grand_canyon_nps/6705376193/in/photostream/

Reference: https://bit.ly/2REaVCr

The Kwagunt formation and the Cryogenian The next unit up in the Grand Canyon is (I think) seen here; the Kwagunt Formation. This rock unit isn’t labeled but from the description and the rock type I think I’ve got the unit right. This is the 2nd part of the Chuar Group, two units of rocks made up of sediments shed off the Laurentian continent as the supercontinent Rodinia broke up. Both units in the Chuar group have a feature that may very well be related to big changes happening in the world. These units contain cycles of shales and limestones, possibly related to changes in sea level and temperature. Some authors have calculated the rate sediments were deposited in these basins and estimated the age of each cycle to be close to 100,000 years – roughly the same duration that we see in the modern glacial-interglacial cycle driven by changes in Earth’s orbit.  There is another really important layer in the Kwagunt formation; in 2000, scientists from the University of New Mexico found an ash layer at the top of the unit that allowed the unit to be dated as ending 742 million years ago. This is a time period in the late Precambrian (also called the Neoproterozoic) when big things were happening. There are few age constraints on sedimentary rocks of these successions, but where there are, ages around 740 million years ago occur just before one of the major glacial deposits found in rocks worldwide; the Sturtian glaciation. The Sturtian glaciation is one of the two hypothesized “Snowball Earth” events, when some scientists believe the Earth may have partially or even totally frozen over for up to 10 million years. That’s where the name of this time period, the “cryogenenian”, comes from; it was probably a pretty cold planet. In this unit therefore we have indications of 100,000 year cycles of sea level and temperature change, as well as chemical changes seen in other units around the world that happen just before the start of the huge glaciation. The cycles in this sequence then might be telling a story of glaciers at the poles growing and shrinking in the few million years before the glaciers truly got out of control. -JBB Image credit: Alan English (creative Comons) https://www.flickr.com/photos/alanenglish/2043920291/ Previous articles: https://www.facebook.com/photo.php?fbid=71718732167564 https://www.facebook.com/photo.php?fbid=717596974968016 https://www.facebook.com/photo.php?fbid=718487278212319 https://www.facebook.com/TheEarthStory/posts/718917208169326 https://www.facebook.com/TheEarthStory/posts/719035941490786 https://www.facebook.com/TheEarthStory/posts/719534524774261 https://www.facebook.com/photo.php?fbid=720485404679173 https://www.facebook.com/photo.php?fbid=720916891302691 https://www.facebook.com/TheEarthStory/posts/721282287932818 https://www.facebook.com/TheEarthStory/posts/721455997915447 https://www.facebook.com/TheEarthStory/posts/722212221173158 https://www.facebook.com/TheEarthStory/posts/722332104494503 https://www.facebook.com/TheEarthStory/posts/723288294398884 https://www.facebook.com/TheEarthStory/photos/a.352867368107647.80532.352857924775258/723925267668520/?type=1 Sources: http://gsabulletin.gsapubs.org/content/84/4/1243.short http://www.sciencedirect.com/science/article/pii/S0012821X09007213 http://www.sciencedirect.com/science/article/pii/S0037073801000872# http://geology.gsapubs.org/content/28/7/619.short

The Nankoweap formation. I’m a little bit frustrated at the quality of pictures available of this formation. The lower units in the Canyon produced a number of photos with clear documentation from where they came from. This formation…this is the best I’ve got.  The Nankoweap formation can be spotted near the lower left in the wide-angle view of the Canyon. See the black unit that has faulted downwards? That’s the Cardenas basalt. The flatter layers on top? That’s the Nankoweap formation.  The Nankoweap formation sits on top of the big thick Cardenas basalt. The boundary between the two units is an unconformity; there are places where the basalt was eroded before sediments started depositing on top. There are also faults which cut through portions of the Cardenas basalt but do not cut the Nankoweap formation, making this unit clearly younger. The rocks of this formation are sandstones with hematite cement, giving them a reddish color. The lower part of the Nankoweap is fairly finely-bedded while the upper portion has coarser beds.  Paleomagneticists have worked on this unit and found that the lower and upper portions of the Nankoweap record strongly different magnetic directions. This change implies that there is a lot of time missing in the middle of the Nankoweap – a disconformity, an unconformity that sits somewhere but doesn’t have major changes in rock type. The Nankoweap is younger than the Cardenas; zircons present in this unit are as young as about 800 million years old. Its hard to tell when Nankoweap deposition started but this unit could potentially represent over 100 million years of time. Like the quartzite below, this rock is composed of sandy sediments derived from erosion of the surrounding areas. There are some pieces of the Cardenas basalt in the Nankoweap, mixed with quartz and feldspar grains. The Nankoweap formation contains a variety of sedimentary structures like mud cracks and ripple marks, indicating that it was deposited in a setting like an arid lake or arid shoreline where the water would appear and disappear as precipitation patterns changed. -JBB Image credits: https://www.flickr.com/photos/grand_canyon_nps/6529420653/ http://en.wikipedia.org/wiki/Cardenas_Basalt#mediaviewer/File:Grand_Canyon_landscape.jpg Sources: http://bulletin.geoscienceworld.org/content/117/11-12/1573.full Previous articles:  https://www.facebook.com/photo.php?fbid=71718732167564 https://www.facebook.com/photo.php?fbid=717596974968016 https://www.facebook.com/photo.php?fbid=718487278212319 https://www.facebook.com/TheEarthStory/posts/718917208169326 https://www.facebook.com/TheEarthStory/posts/719035941490786 https://www.facebook.com/TheEarthStory/posts/719534524774261 https://www.facebook.com/photo.php?fbid=720485404679173 https://www.facebook.com/photo.php?fbid=720916891302691 https://www.facebook.com/TheEarthStory/posts/721282287932818 https://www.facebook.com/TheEarthStory/posts/721455997915447 https://www.facebook.com/TheEarthStory/posts/722212221173158 https://www.facebook.com/TheEarthStory/posts/722332104494503

The first step out of the basement So far I’ve spent a whole lot of time on rocks that only outcrop in narrow areas of the Grand Canyon. The schists, granites, and gneisses exposed in the Canyon’s inner gorge may not show up over much area, but they’re hugely important to understanding the history of the Western U.S. Those schists and granites record the collision of a volcanic arc with the growing North American continent (more commonly called Laurentia) about 1.7 billion years ago. When that collision happened, the rocks now at the bottom of the Canyon were pushed into the Earth, twisted, folded, melted, and metamorphosed. But then, the mountain building stopped.  The next step in our journey up the Canyon isn’t so much a rock unit as it is missing time. This annotated image shows younger rocks from a sequence of sedimentary rocks known as the Grand Canyon Supergroup sitting on top of the schists of the basement.  The Grand Canyon Supergroup is going to be our next step up, but for the moment look at the contact. There’s a random, wavy surface marked as a fault. In some areas the boundary between these rocks may be a fault, in others it is a “nonconformity”. This term is used to describe a boundary between igneous or metamorphic rocks below and sedimentary rocks above where there the surface has been eroded. There must be a period of time missing here; the sediments weren’t deposited on top of the rocks when they were buried deep in the crust, so what happened? There are two key details to stress. First, the surface on top of the schists is an erosional surface in many places. The rocks that were buried deep in the crust somehow got to the surface. Secondly, there is a normal fault seen here; one of several within these units.  To make this sequence, he mountain range above the schists was eroded away and faults helped move material up and down as well. Rocks that were buried beneath 10s of kilometers of mountains got all the way up to the Earth’s surface. This process wasn’t fast. That single layer, the contact between the schists and the overlying sediments, represents nearly 500 million years of missing time. Imagine what it would take to erode away one of the great mountain ranges today; the Rocky Mountains or the Himalayas; this single layer really is that, the complete flattening of a mountain range over half a billion years. Rocks moved up and down on faults and an entire mountain range was eroded away, creating basins and depressions where water could flow in and new sedimentary rocks could be deposited. Those sediments will be the next part of our story. Oh, and the part labeled “Tapeats Sandstone/Great Unconformity”? Don’t worry; we’ll get there eventually. -JBB Image credit and annotation: Dr, Jack B. Share ofhttp://written-in-stone-seen-through-my-lens.blogspot.com/, used with permission. Read more: http://www.indiana.edu/~geol105b/images/gaia_chapter_6/unconformities.htm http://earthphysicsteaching.homestead.com/Principles_Structural_Geology_II.html Previous articles: https://www.facebook.com/photo.php?fbid=71718732167564 https://www.facebook.com/photo.php?fbid=717596974968016 https://www.facebook.com/photo.php?fbid=718487278212319 https://www.facebook.com/TheEarthStory/posts/718917208169326 https://www.facebook.com/TheEarthStory/posts/719035941490786 https://www.facebook.com/TheEarthStory/posts/719534524774261